The present invention relates to a solid lubricant, a production method thereof, a non-asbestos brake friction material, and a sliding component using the solid lubricant. More particularly, the invention relates to a solid lubricant more significantly improved in thermal resistance and oxidation resistance than graphite and enhanced in lubricating performance at high temperature, an effective production method thereof, a non-asbestos brake friction material, and a sliding component using the above-mentioned solid lubricant.
Layered materials such as graphite and molybdenum disulfide and organic materials such as polytetrafluoroethylene (PTFE) are used as solid lubricants in non-asbestos brake friction materials and the other sliding fields. Further, ceramic-treated products of graphite such as C/C composite improved in oxidation resistance and thermal resistance (see Patent Document 1) also begin to be utilized.
However, the non-asbestos brake friction materials are also not sufficiently satisfactory in lubricating characteristics in a high-temperature range of 500° C. or more in the atmosphere. Therefore, the occurrence of wear and abnormal noise are leaded. Accordingly, development of solid lubricants having satisfactory lubricating characteristics in the high-temperature range has become a problem.
By the way, a technique for improving oxidation resistance of graphite by a phosphoric acid or phosphate treatment is a technique widely used in refractories or glosts. However, this is quite irrelevant to improvement of graphite in lubricating characteristics, and is not a technique applied to the solid lubricants.
On the other hand, it has been confirmed that graphite particles treated with aluminum phosphate are improved in wear resistance. However, the lubricating characteristics have not been sufficiently satisfactory in a high-temperature range of 500° C. or more in the atmosphere.
Further, many attempts to coat surfaces of carbon material with SiC (silicon carbide) covering layers excellent in thermal resistance and oxidation resistance have hitherto been made. However, cracks occur in the covering layers due to the difference in thermal expansion between carbon and the ceramic, so that it has been impossible to expect a stable effect. Patent Document 2 proposes a method of implanting boron ions in a surface of a carbon material by a plasma immersion ion-implantation method, thereby forming a modifying layer containing boron carbide to improve adhesiveness of the carbon material, and further forming a SiC covering layer by a CVD method, thereby improving oxidation resistance of the carbon material in a high-temperature range. However, there is no description for a use as a friction material, so that it is unthinkable that this method can be necessarily applied to the friction material.
[Patent Document 1] Japanese Patent Publication Number 4-254486
[Patent Document 2] Japanese Patent Publication Number 2001-106585